The update to 4.0.1 isn't working.
When I go to download a theme, I get the message "Not available for Firefox 3.5.5" The thing is, I already upgraded to 4.0.1
When I go to FireFox > Help > About Firefox, it says my version is 4.0.1. I hit check for updates just in case, but don't get anything.
Wšě wotmołwy (1)
This site thinks you are using 3.5.5, so something is definitely a little bit off.
It is possible to override the true version number with a hidden preference. To clear that:
(1) Type or paste about:config in the address bar and press Enter. Click the button promising to be careful.
(2) In the filter box, type usera and pause for Firefox to filter the list.
(3) Right-click and reset any bolded lines that seem to have wrong version information.
Also, this article lists some additional steps you might need:
Websites or add-ons incorrectly report incompatible browser | Troubleshooting | Firefox Help